1. Field of the Invention
The present invention pertains to a clamped tool having a cutting insert releasably secured to a tool holder, and in particular to the improvement in an assembly for clamping a cutting insert with a mounting bore on the tool holder.
2. Prior Art
FIGS. 4 to 6 depict a conventional insert clamped tool which comprises a tool holder 2 having an insert-receiving seat 1 or recess formed at its forward end and a pin-receiving hole 3 opening to the insert-receiving seat 1. A throwaway cutting insert 4 with a mounting bore 4a formed therethrough is received on the insert-receiving seat 1, and a positioning pin 5 is received in the pin-receiving hole 3 so that its head portion 5c protrudes from the seat 1 to engage with the mounting bore 4a of the insert 4. A clamp 7 is disposed at a position shifted rearward of the tool holder 2 relative to the pin 5, and a clamp plate 8 is interposed between the clamp 7 and the seat 1 with its rear wedge face 8a being held in abutment with a complementary wedge face 7aformed at a rear portion of the clamp 7. A clamp bolt 6 is inserted through the clamp 7 and the clamp plate 8 and screwed into the tool holder 2.
The positioning pin 5 has an externally threaded portion 5a screw-fit in an internally threaded portion 3a of the pin-receiving hole 3, an engaging pin portion 5b fit in a straight portion 3b of the pin-receiving hole 3, and an insert engaging portion 5c held in engagement with the mounting bore 4a of the insert 4. The positioning pin 5 is thus subject to the force caused from the clamp plate 8 to prevent the insert 4 from moving along the insert-receiving seat 1.
In the clamped tool described above, when the clamp bolt 6 is tightened, the insert 4 is pressed by the clamp 7 against the insert-receiving seat 1, and the clamp plate 8 is pressed forward of the tool due to a wedging effect, caused between a head 6a of the clamp bolt 6 and the clamp 7, and a wedging effect, caused between the wedge face 7a of the clamp 7 and the wedge face 8a of the clamp plate 8. Thus, the insert engaging portion 5c of the positioning pin 5 and the mounting bore 4a of the insert 4 are brought into abutting engagement with each other, so that the movement of the insert 4 along the insert-receiving seat 1 can be prevented.
In the aforesaid conventional clamped tool, the inner diameter d.sub.1 of the straight portion 3b of the pin-receiving hole 3 is set somewhat greater than the diameter d.sub.2 of the engaging portion 5b of the positioning pin 5. Therefore, when clamping the insert 4, the positioning pin 5 undergoes a pressing force from the clamp plate 8 and is caused to move within the pin-receiving hole 3. As a result, the position of a cutting edge 4b of the insert 4 in a longitudinal direction of the tool varies during the clamping operation, and hence it has been difficult to position the cutting edge 4b in place.
Furthermore, inasmuch as the inner diameter d.sub.1 of the straight portion 3b of the pin-receiving hole 3 is greater than the diameter d.sub.2 of the engaging pin portion 5b of the positioning pin 5, the straight portion 3b and the engaging pin portion 5b contact each other only at a point or a line. Therefore, the pressing force caused from the clamp plate 8 concentrates on the contact portions, exerting a strong bending force on the positioning pin 5 which can damage it.
Moreover, when the difference between the inner diameter d.sub.1 of the pin-receiving hole 3 and the diameter d.sub.2 of the positioning pin 5 is set small, it sometimes happens that the positioning pin 5 cannot be inserted into the pin-receiving hole 3. On the other hand, if the above difference is great, the amount of the movement of the positioning pin 5 tends to become large, making the disadvantage described above more crucial. Accordingly, the tolerance for the difference between the diameters d.sub.1 and d.sub.2 must be strictly set to a desired value, resulting in a lowering of yields and an increase in machining cost.